The question whether K⁺ depolarization is an appropriate experimental substitute for the physiological nutrient-induced depolarization of the beta cell plasma membrane was investigated using primary mouse beta cells and islets. At basal glucose 40 mM K⁺ induced a massive monophasic response, whereas 15 mM K⁺ had only a minimal insulinotropic effect, even though the increase in the cytosolic Ca²⁺ concentration ([Ca²⁺]_i) was not inferior to that by 20 mM glucose. In voltage-clamp experiments Ca²⁺ influx appeared as nifedipine-inhibitable inward action currents in the presence of sulphonylurea plus TEA to block compensatory outward K⁺ currents. Under these conditions 15 mM K⁺ induced prolonged action currents and 40 mM K⁺ transformed the action current pattern into a continuous inward current. Correspondingly, 15 mM K⁺ led to an oscillatory increase, 40 mM K⁺ to a plateau of [Ca²⁺]_i superimposed on the [Ca²⁺]_i elevated by sulfonylurea plus TEA. Raising K⁺ to 15 or 40 mM in the presence of sulfonylurea (+/- TEA) led to a fast further increase of insulin secretion. This was reduced to basal levels by nifedipine or CoCl₂. The effects of 15 mM K⁺ on depolarization, action currents and insulin secretion were mimicked by adding 35 mM Cs⁺, those of 40 mM K⁺ by adding 35 mM Rb⁺, in parallel with their ability to substitute for K⁺ as permeant cation. In conclusion, the alkali metals, K⁺, Rb⁺ or Cs⁺ concentration-dependently transform the pattern of Ca²⁺ influx into the beta cell and may thus generate stimuli of supraphysiological strength for insulin secretion.